The present invention relates to an apparatus and methods for eliminating or substantially inhibiting electrostatic deposition of charged particles from the coolant onto the surface of an inlet-mixer of a jet pump forming part of a water recirculation system in a boiling water nuclear reactor, and for inhibiting stress corrosion cracking of the metallic parts. This invention particularly relates to an insulating barrier coating that eliminates or substantially inhibits the interaction between the conductive metal housing of the inlet-mixer of the jet pump assembly and the ionic particles in the fluid.
In a boiling water nuclear reactor, an annular space is defined between the core shroud and the reactor pressure vessel wall. Jet pumps are located in the annular space for recirculating coolant through the reactor. The recirculation system circulates the cooling medium around the nuclear reactor core. Jet pumps, which contain no moving parts, provide an internal circulation path for the core coolant flow. Typically, a substantial number of jet pumps, for example, on the order of sixteen to twenty-four, are installed in this annular space. Each jet pump assembly consists of a riser assembly, a riser brace, two inlet-mixer assemblies, and two diffuser assemblies. The inlet-mixer includes a nozzle and a suction inlet. The nozzle may have one orifice or five orifices, depending on the jet pump design. The top of the inlet-mixer is mechanically clamped to the top of the riser transition piece, while the exit end of the inlet-mixer fits into a slip joint with the top of the diffuser. The inlet-mixer is a removable component.
A recirculation pump, external to the reactor vessel, pulls suction from the downward flow of coolant in the annular space. The coolant is pumped to a higher pressure, and is distributed through a manifold to the jet pumps, where the coolant flows in an upward direction through the risers. The coolant splits in the transition piece and changes direction. It is then accelerated in a downward direction through the nozzles and into a mixer section of the jet pump. The nozzles cause a high velocity coolant flow that is approximately one third of the core flow and discharges into the inlet-mixers. Momentum causes surrounding water in the downcomer region of the annulus to also enter the mixer section where it mixes with the outflow from the nozzles for flow through the mixer section and diffuser. This combined flow discharges into the lower core plenum. The coolant then flows upward between the control rod drive guide tubes and is distributed for flow along individual fuel rods inside the fuel channels.
Over time, contaminants tend to accumulate on the inside surface of the inlet-mixers including the jet pump nozzles, forming a layer of “crud.” There is also potential for stress corrosion cracking along these surfaces. The build-up of “crud” is attributed, at least in part, to charged particles suspended in the coolant which interact with the metallic inner surface of the inlet-mixer inducing a triboelectrostatic charge on the surface. This charge creates an electrostatic potential that attracts the suspended particles in the fluid to the metallic surface where they form a layer of particle contaminants. The greatest deposition of “crud” tends to be observed in areas that experience a high velocity flow rate.
The accumulation of the layer of “crud” will tend to degrade the performance of the recirculation system. If the accumulation is excessive, this degradation will affect the efficiency of the plant because the recirculation pumps must be run at a higher speed to maintain core flow. Degradation of jet pump performance can also result in extreme jet pump vibration and damage to jet pump components. Eventually, the inlet-mixer must be mechanically cleaned or replaced during regular maintenance and refueling outages. This process is expensive and time consuming. Consequently, it is important that the accumulation of this layer of “crud” be suppressed or substantially eliminated in order to preserve a clear flow path and maintain the performance of the recirculation system.
In the past, cleaning processes have been proposed that remove the “crud” layer from the inside surface of the inlet-mixer. These processes require removal of the inlet-mixer from the reactor for cleaning in the fuel pool. This is typically accomplished at regular scheduled shutdowns of the reactor, at which times the necessary maintenance is performed. A process using an electrical circuit has also been proposed for reducing the electrostatic deposition of charged particles on the inlet-mixer surfaces that are exposed to the free stream electrical potential in U.S. Pat. No. 5,444,747. This process employs a DC circuit with an active element feedback loop that adjusts the surface potential of the inlet-mixer to minimize the net flux to the inner conducting surface of the parts and thereby reduces particulate deposition. Implementation of this process, however, requires significant attention and maintenance and adds to the overall complexity of the recirculation system.
Accordingly, there remains a need for apparatus and methods of protecting the inlet-mixers of the jet pumps from contaminant build-up. Furthermore, there remains a need for a solution to the problem of “crud” build-up which gradually degrades their performance and requires the need for periodically cleaning and maintaining the jet pump.